1. Field of the Invention
The invention pertains to the field of semiconductor fabrication. More particularly, the invention pertains to methods of crystallizing amorphous silicon film and methods of forming thin film device structures such as thin film transistors and thin film solar cells incorporating the silicon film formed using such crystallization process.
2. Description of Related Art
Polycrystalline silicon (polysilicon) thin films are used in devices such as thin film transistors (TFTs) or solar cells. The polysilicon TFT arrays are used as backplanes for switching liquid crystal displays (LCDs) and for driving organic light emitting diode (OLED) displays. For the LCD applications, polycrystalline TFTs are used instead of more popular amorphous silicon (a-Si) TFTs when the peripheral driving circuit is also made using TFTs, since polysilicon TFTs have about two orders of magnitude higher carrier mobility compared to a-Si TFTs. Additionally, for higher resolutions LCDs, polysilicon TFTs are preferred over a-Si TFTs. For OLED applications, currently the use of polysilicon TFTs is the only practical way to make reliable displays, as instability of a-Si TFTs makes long term OLED operation difficult.
In polysilicon TFTs, the active layer (channel of TFT) is made of polycrystalline silicon. During the fabrication of polysilicon TFTs, the channel layer is usually deposited as a-Si and then it is subsequently annealed to convert it to polycrystalline silicon. This process is referred to as crystallization. The crystallization of a-Si needs to be performed at a thermal budget (temperature/time budget) lower than that which can damage the glass substrate used for the display. The most commonly used method for crystallization in the industry today is excimer laser annealing (ELA), as thermal budgets encountered during the ELA do not cause damage the glass substrate. Although the ELA process is the most commonly used method, it has several disadvantages. First of all, the ELA process is expensive in terms of cost of equipment, its operation and maintenance. Secondly, since the ELA is performed by scanning a pulsed laser beam, there is a non-uniformity in TFT characteristics resulting from pulse to pulse variation of the laser beam. The scanning non-uniformity is visible on an image of an OLED display in the form of scan lines. Additionally, there is a high surface roughness for the polycrystalline silicon layer formed using ELA. Laser annealing is not suitable for creating thicker polycrystalline films (several thousand angstroms to several micrometers) that are needed in solar cells, because laser annealing does not efficiently produce these films.
The least expensive and simplest crystallization process for a-Si is thermal annealing and it is known as solid phase crystallization (SPC). However, the thermal budgets needed to crystallize a-Si by SPC are too high to be practical for mass production of TFTs. For example, for a-Si films deposited at about 250° C. by a PECVD method, the annealing time needed to crystallize the films at 600° C. is about 15 hours. Such times are too long for mass production of devices.
The annealing times for crystallization can be reduced exponentially by increasing the temperature. For example, for the same a-Si film mentioned above, the crystallization time at 650° C. is about 80 minutes and, at 700° C., it is of the order of 10 minutes. However, the glass substrate used for these TFTs can easily bend at these thermal budgets.
In order to reduce the thermal budget for crystallization of a-Si, people have deposited very thin (10-30 angstrom) Ni or Pd films on the a-Si surface and crystallized it by a process called metal induced crystallization (MIC) at a thermal budget 100 to 150° C. lower than those needed during SPC. The crystallization in this case proceeds by formation of a crystalline silicide (example Nickel silicide). This method is very attractive because of its lower thermal budget, but during the annealing, there is an incorporation of the metals and/or their silicides into the entire silicon layer, which affects the device characteristics adversely, especially the leakage current, which increases significantly for these devices.
The amorphous silicon film deposited on a substrate such as glass can be selectively heated by generating an alternating or varying magnetic field by introducing alternating electrical current in an induction coil in the vicinity of the amorphous silicon film (U.S. Pat. No. 6,747,254, issued Jun. 8, 2004, entitled “Apparatuses for heat-treatment of semiconductor films under low temperature”, incorporated herein by reference). This is done to keep the substrate at a temperature lower than the a-Si crystallization temperature, while the a-Si film is at a high enough temperature to crystallize. However, heating of the film due to the magnetic field strongly depends upon the conductivity of the film. Since the conductivity of amorphous silicon is very low (could be as low as 10−12 S/cm), the magnetic field is unable to effectively heat the a-Si film. Alternatively, a conductive susceptor can be placed under the substrate and heated by the magnetic field, but this has a disadvantage of the substrate being heated by the susceptor.